1. Field of the Invention
The present invention relates to an RF power sensor fabricated in a Micro Electro Mechanical System (MEMS). More particularly, the present invention relates to an RF power sensor capable of measuring an RF signal power using capacitance.
2. Description of the Related Art
Micro Electro Mechanical System (MEMS) is a technology for implementing mechanical parts into electrical devices using a semiconductor process. Each device, which has a mechanical structure and operates accordingly, is manufactured through a semiconductor manufacturing process on a semiconductor wafer.
Precise power measurement is essential for high quality connections in communications systems. In general, terminals or base stations transmit power to an extent that signals reach a target. However, power transmissions above that which are necessary cause unnecessarily high power consumptions and are restricted by regulations regarding maximum allowable transmission values. Therefore, a possible solution to the above problem includes using a power sensor to precisely measure RF power.
In general, there are two types of sensors used for RF power measurements. A first type is a rectification-type sensor (rectifier) and a second type is a thermal-type sensor (thermal detector).
A rectification-type sensor uses a frequency down-conversion process converting a high frequency to a low frequency through characteristics of a non-linear device, such as a diode, to take RF power measurements.
A thermal-type sensor uses resistance changes or dielectric permittivity changes due to temperature changes to take RF power measurements. In addition, there are several types of thermal-type sensors. A thermal-type sensor may be a thermistor, a thermocouple sensor, or the like. Unlike the rectification-type sensor, the thermal-type sensor has an advantage of easy measurements due to the absence of super high frequency characteristics of a detection signal. Another advantage of the thermal-type sensor is that cooling is not necessary due to the operation capability at room temperature with a simple interface.
The power sensor of a thermistor uses resistance changes of the thermistor capable of sensing temperature changes of an endmost load based on RF power to calculate an input power. The thermistor changes a resistance value thereof as a temperature thereof changes. The relation between the resistance and power of the thermistor is non-linear and different depending upon devices for which the thermistor is used so that the thermistor is used in a self-balancing bridge circuit. The sensor of the thermistor has a relatively high degree of precision and may be used in association with other sensors.
The dynamic range, however, is restricted to a range from about −10 dBm to −20 dBm, the values of which indicate a relatively narrow range in comparison to the present RF technology standard. Further, the thermistor is not widely used since, as a practical matter, the thermistor does not meet requirements for digital communications due to a slow response speed caused by the use of temperature changes.
Further, the power sensor of a thermocouple is appropriate for measurements of diverse kinds of RF power up to complicated digital phase modulation signals due to the capability of responses to real power.
The thermocouple power sensor produces a small voltage due to a voltage difference between two kinds of metals having a temperature difference. That is, the thermocouple power sensor generates a small voltage difference so that a plurality of thermocouples connected together in series are used to form a thermopile. The thermopile produces a much larger output to enable an electric current sensing circuit to be simply structured. A dynamic range of the thermocouple power sensor is from −30 dBm to +20 dBm, a wide bandwidth of 20 GHz, and of excellent sensitivity. However, a thermocouple power sensor has a drawback to response speeds as slow as a few msec as in the thermistor power sensor.
In the meantime, a diode power sensor rectifies RF energy to produce a DC voltage without measurements of heat generated from an RF signal. The diode power sensor has good sensitivity compared to other kinds of sensors and a wide dynamic range from −70 dBm to −20 dBm. Further, the diode power sensor has a wide bandwidth of 40 GHz and response speeds as fast as a few μsec. Therefore, the diode power sensor is appropriate for the Time Division Multiple Access (TDMA) method, which requires fast response characteristics.
However, in a case of the diode power sensor, terms of high order are increased for power over −20 dBm so that non-linear characteristics occur.
In the meantime, in a case of the diode power sensor, there are two methods for measuring power over +20 dBm. The first method is to place an attenuator in the stage prior to a diode, which has a drawback of deteriorating the degree of precision of a power sensor. The second method is to calibrate a signal for use, which uses the diode calibration and digital signal processing (DSP) so that it has a drawback of a high cost.
As above, the conventional power sensors have problems in that the characteristic impedances thereof change based on temperature changes, which causes difficulties in matchings and deteriorates measurement precision degrees since they need couplers, attenuators, and the like.